1. Field of the Invention
The present invention relates to tools used by electricians, electrical contractors and the like, for handling flexible metal conduit, of the type used to protect and guide electrical wires used in buildings and other structures. More particularly, the invention relates to a tool for temporarily and securely joining a length of flexible conduit to a flexible pulling cable, to facilitate pulling the conduit through passageways in a structure.
2. Description of Background Art
Flexible metal conduit is used extensively in residential, commercial and industrial buildings and other structures. Such conduit is used to protect electrical wiring within the conduit, from mechanical impact damage and/or the elements.
Typical flexible metal conduit consists of long continuous lengths of thin-wall, hollow flexible metal tubing. Typical standard inner diameters for such tubing are one-half inch or three-quarters of an inch. The tubing is made flexible by fabricating it from long strips of metal which are curved convexly outwards, and formed into a helix in which the adjacent edges of helical loops are interlocked, but free to move a certain extent with respect to one another. Thus formed, flexible metal conduit has convolute inner and outer wall surfaces, each of which has a continuous helical indentation.
In typical applications in which flexible metal conduit is used, construction of a building or other structure is substantially near to completion before electrical wiring is installed. Therefore, it is usually necessary to install wiring, and the flexible metal conduits used to protect the wiring, by working the flexible wires and conduit through labyrinthine or serpentine passageways between walls, studs, joists, beams or other structural members of a building. Working or "snaking" flexible wires or conduit through passageways of a building requires some degree of effort and skill, and certain techniques have been developed to facilitate the process.
First, a thin, flat, flexible steel strip, somewhat like a very long tape measure, is threaded into a desired entrance or exit hole for electrical wiring, such as through a wall opening in a baseboard, near a ceiling lighting fixture, or near a wall switch. Then, the flexible fish tape is snaked through passageways between studs in the walls of a building until a flexible cable leader attached to the remote end of the fish tape exits through a desired exit or entrance opening. The distal end of the leader is then attached to one or more electrical wires or cable. Finally, the near or proximal end of the fish tape is grasped and pulled, pulling the attached wires or cable through the serpentine passageways defined by the original outgoing passage of the fish tape. Eventually, the end of the wire or cable attached to the fish tape exits through the hole into which the fish tape leader was initially inserted, allowing the wires to be connected to a desired switch or fixture.
Pulling a fish tape through a passageway with wires or cables attached to the fish tape requires a substantial amount of force. This is because the wires must be dragged along the passageways in contact with various frame members of the structure, and with insulation. Thus, a substantial amount of frictional forces are usually experienced in the process. Because of the high forces which are required to pull a fish tape and attached wires, a number of tools have been disclosed which permit a high pulling force to be exerted, while minimizing possible damage to the fish tape. An example of such a tool is found in U.S. Pat. No. 4,746,099, May 24, 1988, Lopes, Cam Action Fish Tape Puller.
The distal end of a fish tape leader may also be attached to one end of a length of flexible metal conduit, to permit the conduit to be pulled through a serpentine passageway within a structure, the passageway having been defined by the initial path of the fish tape, as described above.
Now, the flexible conduit obviously must be larger in diameter than the electrical wires and/or cables which it is intended to enclose. Also, the outer wall surface of flexible conduit is convolute, as described above, rather than being smooth like the outer surface of typical electrical wire or cable. Therefore, it usually requires a substantially larger force to pull conduit through a passageway than is required to pull wire or cable along the same path. For that reason, any attachment device used to attach a fish tape leader to the end of a flexible conduit which is to be pulled by the fish tape must be capable of transmitting a substantial tensional force. Also, it would be desirable for such an attachment device to be quickly and easily attachable to and removable from a length of flexible conduit.
A number of devices have been disclosed which are related to the task of pulling flexible members through serpentine passageways. These include the following U.S. Patents:
Kepler, U.S. Pat. No. 1,672,324, June 5, 1928, Wire threading Leader and Coupler.
Bergman, U.S. Pat. No. 2,339,671, Jan. 18, 1944, Cable Puller.
Battaglia, U.S. Pat. No. 4,078,767, Mar. 14, 1978, Wire Pulling Apparatus.
Breck, et al., U.S. Pat. No. 4,659,126, Apr. 21, 1987, Duct Pulling Tool, discloses a multi-element tool for engaging the inner surface of convolute flexible conduit. The present inventor is unaware of any conduit pulling attachment which has a unitary, or one-piece, conduit-engaging fitting.